The present invention relates to a relief mechanism which, when pressure in one of a pair of supply/discharge channels connecting a hydraulic actuator to a changeover valve becomes high, permits fluid to escape from the channel to the other channel, as well as to a hydraulic circuit equipped with such a relief-mechanism.
For example, a relief mechanism such as that disclosed in Japanese Utility Model Application Laid-open No. Hei-5-94504 (filed by the applicant of the present invention) has been known as an existing relief mechanism. As shown in FIG. 3, the existing relief mechanism comprises a pair of supply/discharge channels 13, 14 connecting a hydraulic motor 11 to a changeover valve 12; a connection channel 15 connecting together the supply/discharge channels 13, 14; a first relief valve 17 provided at some point in the connection channel 15; a second relief valve 20 provided at some point in the connection channel 15 in shunt with the first relief valve 17; and a cylinder 22. More specifically, for example, when the internal pressure of the supply/discharge channel 13 reaches a relief pressure level, the first relief valve 17 is opened and permits fluid to escape from the supply/discharge channel 13 to the other supply/discharge channel 14. In contrast, when the internal pressure of the supply/discharge channel 14 decreases to a given pressure level lower than-the relief pressure level, the first relief valve 17 is opened only during the time period over which the fluid is flowing to a first intermediate channel 16 from the supply/discharge channel 14, to thereby permit fluid to escape from the supply/discharge channel 14 to the supply/discharge channel 13. When the internal pressure of the supply/discharge channel 14 reaches the relief pressure level, the second relief valve 20 is opened, to thereby permit fluid to escape from the supply/discharge channel 14 to the supply/discharge channel 13. In contrast, when the internal pressure of the supply/discharge channel 13 decreases to a given pressure level lower than the relief pressure level, the second relief valve 20 is opened only during the time period over which fluid is flowing to the supply/discharge channel 14 from the supply/discharge channel 13. The cylinder 22 connects the first intermediate channel 16 to a second intermediate channel 19. When a piston 21 housed therein is moved to a stroke end as a result of being pressed by the fluid flowed from the first and second intermediate channels 16, 19, the cylinder 22 stops the influx of the fluid from the first and second intermediate channels 16, 19.
However, since the flow area of the first and second intermediate channels 16, 19 and the displacement of the cylinder 22 is constant in the foregoing existing relief mechanism, the injection-valve opening pressure and the injection-valve opening time of the first and second relief valves 17, 20 are limited to constant values.